1. Field of the Invention
The present invention relates to a mobile station, a base station, a communication system, and a communication method.
2. Description of the Related Art
Conventionally, in a wideband-code division multiple access (W-CDMA) system, consecutive high quality communication is performed through soft handover. Soft handover refers to a mobile station transmitting/receiving a plurality of signals in a plurality of sectors or cells having different spreading codes.
Meanwhile, in the case of soft handover in a downlink, a plurality of base stations transmits dedicated physical channels of the same information. Therefore, interference increases. As a result, in a downlink, site diversity effects obtained through soft handover are less than those in an uplink. Accordingly, site selection diversity transmission power control (SSDT) method has been proposed (“SSDT-Site Selection Diversity Transmission Power Control for CDMA Forward Link”, H. Furukawa, K. Hamabe, and A. Ushirokawa: IEEE Journal on selected areas in communications, vol. 18, no. 8, pp. 1546-1554, August 2000). SSDT method is a method where, among the base stations performing soft handover, only the base station with the largest received power or the largest signal to interference power ratio of a received signal (hereafter referred to as the “most appropriate base station”) performs data transmission and this most appropriate base station switches to high speed mode.
Considering the rapid spread of the Internet, increases in the dimension of information, increases in the capacity of information, and the development of the next generation Internet in recent years, there is a pressing need in mobile communication systems to develop a radio access scheme capable of implementing an information transmission rate exceeding 2 Mbps. This need is considered to be increasing, especially in downlink traffic requiring high speed and a large capacity, including downloads of images, files, or moving images such as video from a database or Web site. Therefore, a high-speed packet transmission technique suitable for high speed/large capacity traffic is indispensable.
From a background such as this, a proposal has been made to implement high speed packet transmission with a maximum information transmission rate of 2.4 Mbps based on an IS-95 radio interface (“CDMA/HDR: A Bandwidth-Efficient High-Speed Wireless Data Services for Nomadic Users”, P. Bender, P. Black, M. Grob, R. Padovani, N. Shindhushyana, and A. Viterbi: IEEE Communication Magazine, Vol. 38, no. 7, pp. 70-77, July 2000). Additionally, in the 3rd generation partnership project (3GPP), implementation of high-speed packet transmission having a maximum information transmission rate of approximately 10 Mbps obtained by expanding the W-CDMA radio interface has been studied.
Application of adaptive modulation and channel coding based on adaptive radio link control such as channel coding, has been studied for such high speed packet transmission, for example, as proposed in “Symbol Rate and Modulation Level-Controlled Adaptive Modulation/TDMA/TDD system for High-Bit-Rate Wireless Data Transmission”, T. Ue, S. Sampei, and N. Morinaga: IEEE Transactions VT, pp. 1134-1147, Vol. 47, no. 4, November 1988).
In adaptive modulation and channel coding based on adaptive radio link control, the data modulation level, spreading factor (SF), the number of multi-codes, and the channel coding rate are switched over according to the propagation environment of a user in order to conduct high speed data transmission efficiently. For example, as for data modulation, the quadrature phase shift keying (QPSK) modulation used in the current W-CDMA is switched over to a multi-level modulation having a higher efficiency, such as 8 PSK modulation, 16 quadrature amplitude modulation (QAM), or 64 QAM, as the propagation environment becomes favorable. As a result, the maximum throughput of the communication system can be increased.
As for the high speed packet transmission, application of the automatic repeat request (ARQ) technique proposed in “Automatic-Repeat-Request Error Control Schemes”, (S. Lin, D. Costello, Jr., and M. Miller: IEEE Communications Magazine. Vol. 12, no. 12, pp. 5-17, December 1984) has also been studied.
The high-speed packet channel used for such high-speed packet transmission is a shared channel. The shared channel is used by a plurality of mobile stations. Therefore, the transmission power of the shared channel becomes significantly larger than that of the dedicated physical channel. The dedicated physical channel is a dedicated channel for each mobile station.
Accordingly, in SSDT method, where it is always only the most appropriate base station that is transmitting data, interference with other cells or sectors can be reduced in comparison with a soft handover where there is a plurality of base stations performing data transmission simultaneously. As a result, with high-speed packet transmission, application of the SSDT method is studied in order to improve data throughput in the end of cells and sectors, and enlarge the range capable of achieving target data throughput value.
However, in high-speed packet transmission that uses the high-speed packet channel, each base station assigns the high-speed packet channel to a plurality of mobile stations. The high speed packet channel is a shared channel. Therefore, an opportunity of communication using a high-speed packet channel is assigned only to one, or a few mobile stations at any point in time.
Accordingly, even if the most appropriate base station is selected based on the received power or signal to interference power ratio of the received signal, and the cell or sector covered by the most appropriate base station is selected as well, the opportunity of communication is not always assigned to the mobile station in that cell or sector upon receiving assignment of the high speed packet channel. In particular, when there is a large traffic load in the selected cells or sectors, the opportunity of communication assigned to the mobile station in that cell or sector becomes significantly limited. In other words, the frequency with which the base station grants the opportunity of communication to mobile stations drops. In addition, throughput of the mobile station to which the opportunity of communication is not assigned drops. Furthermore, since the base station selects a cell or sector for a plurality of mobile stations based on the received power and signal to interference power ratio in the mobile station, control load in the base station increases. Therefore, the efficiency with which the base station performs control in order to communicate with the mobile stations also drops. As a result, in particular, data throughput performed by the mobile stations located at the end of the cells or sectors drops. Accordingly, the range capable of achieving the target data throughput value cannot be enlarged.